1. Field of the Invention
The present invention relates to a vanadium-containing polymerization catalyst that is especially useful in low temperature, low pressure ethylene polymerization processes. The catalyst of the present invention produces polymer at a high level of catalyst activity. The catalyst of the present invention is designed to provide a high level of response to hydrogen as a molecular weight control agent. The subject catalyst further produces polymer that has a bimodal molecular weight distribution. Compositionally, the catalyst of the present invention comprises a solid inorganic oxide or inorganic phosphate (or mixtures thereof), and compositions of zinc and vanadium.
2. Description of the Prior Art
Vanadium-containing compositions have for some time been proposed for use in polymerization catalyst systems. Such catalysts, however, have had relatively little use on a commercial scale since the polymers produced therewith were produced at a relatively low level of catalyst activity and/or the catalysts produced polymer with poor morphology and/or had the tendency to foul the reactor during polymerization. Accordingly, vanadium-containing catalyst systems have not achieved widespread use in polymerization--particularly olefin polymerization--as have titanium or chromium based catalysts. The development of commercially useful vanadium-containing polymerization catalysts has been particularly elusive despite the advantages attendant these systems, such as easier recovery and better handleability of the catalyst as well as greater control over the shape and size of the polymer particle and the overall superior quality of the polymer particle thus produced.
Examples of the development of vanadium-containing catalyst systems include: U.S. Pat. No. 3,786,032 which describes a catalyst used to polymerize vinyl and vinylidene halides, the catalyst being formed from an inert solid matrix, vanadium compounds and an organo-zinc complexed with an oxime. U.S. Pat. No. 4,559,318 discloses a vanadium dihalide complexed with an ether, such as tetrahydrofuran, and a zinc compound. The catalyst includes a porous support. U.S. Patent No. 4,611,038 describes an olefin polymerization catalyst system prepared from a vanadium metal component, a porous support, and an organo-zinc compound; to yield the active solid hydrocarbon catalyst system, a controlled amount of prepolymerization takes place with this composition.
A more recent attempt to produce a vanadium-containing catalyst composition that produces alpha olefin polymers is described in U.S. Pat. No. 4,866,021. The catalyst system utilized therein incorporates supported vanadium, aluminum, and zinc compounds and further requires the presence of titanium.
In addition to producing polymers having less than desirable physical characteristics, the catalysts thus recognized are not easily fabricated. In summary, there is a continuing need in the art for a vanadium-containing catalyst that is useful in olefin polymerization.